There is much interest in renewable power generation, particularly fuel cells. A fuel cell is an energy conversion device that invariably comprises two electrodes, i.e., a cathode and an anode, upon which electrochemical reactions occur to enable the transformation of stored chemical energy into electrical energy. Fuel (e.g., hydrogen, methanol) is oxidized at the anode to release electrons that are then routed through an external circuit, while protons are transported through a proton exchange membrane to the cathode. The circuit is completed inside the fuel cell when the electrons are received back again from the external circuit at the cathode, where they combine with protons and oxygen atoms in a reduction reaction to produce water. The output of the fuel cell then is electrical energy and heat, produced by the production of water from protons and oxygen.
Typically, catalysts are incorporated into the anode and cathode electrodes to facilitate fuel oxidation and oxygen reduction. Current preferred catalysts for fuel cells are particulate noble metals; however, these metals are expensive, inherently inefficient, non-renewable and not easily characterized. For these reasons, substitution of noble metals with homogeneous redox catalysts is a desirable goal; but, low current densities (which result in inadequate power and/or volume) have made this approach uneconomical with previously disclosed systems.
Use of enzymatic catalysts permit the incorporation of the redox catalyst into fluidic microdomains and thereby makes higher current densities possible due to: 1.) locally high concentrations of catalyst (since the catalyst is not confined to one monolayer density); 2.) high electron diffusion coefficients; and, 3.) opportunities for convective transport. Addtionally a redox catalyst that is enzymatic can easily use bio-available energy sources such as glucose.
The redox enzyme in an enzymatic biofuel cell participates in an electron transfer chain at the anode by oxidizing the fuel. However, redox enzymes are incapable of direct contact with the electrode since their redox centers are insulated from the conductive support by their protein matrices (Katz et al., “Biochemical fuel cells”, In Handbook of Fuel Cells—Fundamentals, Technology and Applications, 1, Ch. 21 (2003)). In order to bring these enzymes into contact with the electrode and to improve the electron transfer rate, an initially oxidized electron transport redox mediator is used to reoxidize the enzyme. The electrons are then transferred to the anode and the electron transport mediator is once again oxidized. A similar process occurs at the cathode.
Numerous examples of enzymatic biofuel cells are described in the literature for use in sensor applications where it is desirable to use electrodes in vivo at low power. However, immobilization of the redox catalyst and/or electron transport mediator and restricted access of the electrochemical reagents to the redox catalyst immobilized inside the electrode limit the current densities that can be obtained, limiting the commercial practicablity of these biofuel cells. Additionally, the majority of these systems require the presence of a membrane or other barrier, separating the anode and cathode, which compartmentalizes the reductant and oxidant. Such membranes add cost to the fuel cell and limit miniaturization.
In co-pending U.S. patent application Ser. No. 10/932,371 published as US Patent Publication US 2005-0074663, the Applicants teach that the low current densities of a biofuel cell may be overcome by constructing a fuel cell comprising an electrode consisting of a microporous current collector that incorporates a multitude of domains, wherein each domain contains a soluble redox catalyst. Neither the catalyst nor the electron transport mediator are immobilized in any fashion within the domain but are instead in fluid association with one another The fluid association of these elements of the electrode is unique and responsible for a more efficient system than heretofore described. In spite of the advance in the art provided by this system, preferred electrodes of this type still require a membrane or presence of a membrane or other barrier to separate the anode and cathode.
It is well accepted in the art of fuel cell design and manufacture that a membrane or other barrier is required to physically separate the anode and cathode. As such, a tremendous volume of literature exists that describe preferred materials for proton exchange and details of each material's proton transport and conductivity, etc. Recent work by Mano, N. et al. (J Am Chem Soc. Nov. 6, 2003 ; 124(44):12962-12963; Chem Commun (Camb). Feb. 21, 2003; (4):518-519; J Am Chem Soc. May 28, 2003 ; 125(21):6588-94; Proceedings, Electrochemical Society 2003, 2002-25 (Micropower and Micro-devices): 176-182; and WO 03/106966 A-2; see also, Heller, A. “Miniature Biofuel Cells” (Review) Phys. Chem. Chem. Phys. 2004, 6:209-216) has suggested that modifying the conformation of the fuel cell elements may mitigate the need for a membrane per se. Specifically, this group describes miniature membrane-less biofuel cells, that operate at up to 0.78 V under physiological conditions. The anodic electrocatalyst was comprised of a glucose oxidase and an anode redox polymer mixture, crosslinked onto the surface of a carbon fiber in the form of a swollen hydrogel; likewise, the cathodic electrocatalyst was created by a hydrogel of bilirubin oxidase and a cathode redox copolymer crosslinked onto the surface of a second carbon fiber. Although no membrane is used in this system per se, immobilization of the cathode and anode in the hydrogel effectively produced a barrier, separating these elements of the cell.
The work of S. Topcagic and S. Minteer (Abstracts of Papers, 227th ACS National Meeting, Anaheim, Calif., Mar. 28-Apr. 1, 2004; American Chemical Society, Washington, D.C.) teach a membraneless ethanol/oxygen biofuel cell. Specifically, these authors have utilized enzymes at the cathode and anode. However, the enzymes are immobilized and may not be freely accessible to the fuel.
A need exists therefore for the development of a membrane-free biological fuel cell having redox catalyst- or enzymatic redox catalyst-based electrodes that are capable of generating useful current densities.
Applicants have solved the stated problem by the design of a fuel cell electrode that comprises the redox catalyst and substrate in fluid association with each other within a microdomain of the electrode; and when these electrodes are used in a fuel cell, no membrane is required for the separation of the anode and the cathode and requisite components.